parent_category in import/export

This column is for specifying the parent category of the categories of the product. But that column is optional. It's useful when you have two categories with the same name so that the system knows which one you're talking about.

Do you want to remove the products ? Or do you want to remove the link of these products to that category ?
If you want to remove the products of a category, it's easy, you can do that on the listing products with the delete button after selecting all the products of a category.
If you want to remove the link between them the easiest is to use phpmyadmin and run a query like that:
DELETE FROM jos_hikashop_product_category WHERE category_id=XX

If your categories are already created, do not use the parent_category column and directly put the id of the category you want in the "categories" column.

No categories cannot be overwritten with the CSV import.

It's better to use the id, especially if you have several categories with the same name

The product_id will be ignored. However, if you set the product_code of your products in your CSV, the information of the products will be updated.

You can just create such CSV:

product_code,categories
my_prod1,18
my_prod2,20

That will reassign my_prod1 to the category 18 and the my_prod2 to the category 20